Immunomodulatory properties of morphine and the hypothesised role of long-term opioid use in the immunopathogenesis of tuberculosis

Epidemiological studies have shown high tuberculosis (TB) prevalence among chronic opioid users. Opioid receptors are found on multiple immune cells and immunomodulatory properties of opioids could be a contributory factor for ensuing immunosuppression and development or reactivation of TB. Toll-like receptors (TLR) mediate an immune response against microbial pathogens, including Mycobacterium tuberculosis. Mycobacterial antigens and opioids co-stimulate TLRs 2/4/9 in immune cells, with resulting receptor cross-talk via multiple cytosolic secondary messengers, leading to significant immunomodulatory downstream effects. Blockade of specific immune pathways involved in the host defence against TB by morphine may play a critical role in causing tuberculosis among chronic morphine users despite multiple confounding factors such as socioeconomic deprivation, Human immunodeficiency virus co-infection and malnutrition. In this review, we map out immune pathways involved when immune cells are co-stimulated with mycobacterial antigens and morphine to explore a potential immunopathological basis for TB amongst long-term opioid users.


Introduction
Tuberculosis (TB) remains a longstanding global health challenge, claiming 1.5 million lives in 2020.Two-thirds of new cases come from the eight highest-burdened nations, with six of them in South and Southeast Asia.(1)population is latently infected with TB.The risk of reactivation of latent Mycobacterium tuberculosis (MTB) is rising due to international travel, migration, immunosuppressive co-morbidities, and medication use, affecting both developed and developing countries.(2) In order to achieve the World Health Organization goal of reducing 90% of new TB cases by 2035, it is vital to better understand the key causes of reactivation of latent TB.Multiple risk factors for TB are clustered in different subpopulations.(3)(4)(5) Epidemiological data have shown that long-term opioid users are more susceptible to TB than the rest.(6) As per a comprehensive community-based case-control study, higher TB risk was independently associated with tobacco smoking, drug use (especially injectable drugs OR = 5.67; 95%CI: 2.68, 11.98), homelessness and area-level deprivation.The strongest risk factor among the intermediate social determinants was misuse of class A injectable drugs (e.g., Ecstasy, Cocaine, Crack Cocaine, Heroin), with five times higher TB odds (OR = 5.67; 95%CI = 2•68, 11.98) compared to those who never misused class A drugs (adjusted for age, sex, BCG vaccination status and long stays in high TB area).(7) A multivariate analysis has shown that drug use was associated with smear-positive TB (OR 2.2, CI 311 -401, p<0.001).( 8) Even accounting for genetic, environmental, socio-economic, and culture-related risk factors, raises the possibility of an underlying immunopathological basis leading to immune suppression.(6,(9)(10)(11) TB is often the most common opportunistic infection in endemic areas.(11) This highlights the importance of exploring independent pathophysiological mechanisms causing immune impairment in chronic drug use separately in different infections despite the multiple confounding factors causing generalised immune suppression.
The World Drug Report of 2021 has reported that around 275 million people used illicit substances in 2020 globally, highlighting another growing health challenge.Opioids are the most heavily used substance.Over 75% of substance users live in developing countries, where the prevalence of TB is also highest.(12) Longterm use takes different forms, including misuse of prescription opioids, habitual use without dependence, and increased use as a long-term analgesic.(13) A recent review published in the Lancet has highlighted the TB burden among vulnerable groups worldwide with variable prevalence.(14) Table 1 further summarises studies assessing a link between drug use and TB.Except for one study, others point towards a strong link between drug use and TB risk.As multiple confounders co-exist in these vulnerable populations, causality cannot be independently assessed.Epidemiological data assessing TB infection among drug users taking it as an independent variable is a future need.More specific details of the nature of drug use and the immune status will further help fill the missing data gap.
Immunomodulatory properties of opioids vary in the presence of different microbial antigens.Detailed characterisation of immune pathways affected by opioids in the presence of specific microbial pathogens is essential to characterise mechanisms of immunopathogenesis further.(20,21) Due to the widespread testing of morphine in multiple previous pre-clinical studies (22), this review collates the immunomodulatory properties of morphine,  and it suggests a potential role for opioids during chronic use in the immune pathogenesis of TB.

Opioid receptors in the immune system
Opioids act on their receptor (OR), a G protein-coupled receptor (GPCR).They are categorised into two distinct groups: classical OR, which includes morphine (mu) MOR, ketocyclazocine (kappa) KOR, and vas deferens (delta) DOR, and non-classical OR, which includes nociceptin Orphanin FQ peptide receptor (NOR).(23,24) ORs are located in multiple immune cells, including macrophages and lymphocytes, and are widely distributed in the human body.Both endogenous opioid peptides and exogenous synthetic opioids with different molecular properties act on the same receptors, giving rise to variable downstream effects.(21,25) Chronic morphine administration is known to cause impairment of both innate and adaptive arms of the immune response.(26) The concept of direct and indirect morphine action was on the immune system first introduced through work in preclinical studies, which indicated that the MOR mediates morphine-induced immunosuppression and that although some functions are amplified in the presence of cortisol or sympathetic activation.(27) The activation of KOR has also been shown to reduce antibody production, inhibit phagocytic cell activity, inhibit T cell development and alter the production of various proinflammatory cytokines, chemokines, and the receptors for these mediators.(28) In vitro experiments have shown that the DOR agonist KNT-127 causes immune suppression in rat models with colitis.(29) It has further shown the functional differentiation of OR subtypes located on the immune cells responding differently to endogenous and exogenous opioids.The novel M3OR subtype has been characterised as an opioid peptide-insensitive and opiate alkaloid-selective GPCR that is functionally linked to constitutive nitric oxide synthase activation.Opioid peptides stimulate granulocyte and immunocyte activation and chemotaxis via the activation of a novel leukocyte D2OR subtype.However, opiate alkaloid M3OR agonists inhibit these same cellular activities.(30) Chemokine receptors (CCR), which mediate chemokine response, belong to the same class of GPCR as OR and possibly share a common evolutionary origin.The evidence suggests that these receptors cross-desensitise each other, whereas morphine that binds to OR can block CCR signalling and vice versa.(31,32) The mechanisms underlying heterogenous desensitisation could be the formation of receptor heterodimers and protein kinase C-mediated phosphorylation of Serine, Threonine and Tyrosine moieties.Heterogenous desensitisation may be one mechanism of immune suppression by opioids with high doses and long-term exposure.(33,34) Bivalent molecules can be tested to modify the complex and its cellular effects.Different immune effects mediated by OR in the presence of mycobacterial antigens and morphine are discussed in the subsequent sections.Bivalent molecules that selectively modulate the OR-CCR complex have therapeutic potential, such as VZMC013, which targets the MOR-CCR5 heterodimer to inhibit opioid-exacerbated HIV1 entry into the immune cells.Similar molecules may have applications in managing opioid-induced immune suppression, including the potential TB risk (35).

Modulation of the immune cells and mediators by morphine
A focused literature search in Google Scholar, PubMed and Medline was carried out to extract the studies conducted to find the effects of 'morphine' on the 'immune system'.Tables 2, 3 comprehensively summarise the immune cells and mediators influenced by morphine administration.Morphine suppresses multiple immune cells, including macrophages, which play a crucial role in the immunopathogenesis of TB.The consequences

In vivo murine
Suppressed activity in the spleen via a neuron-mediated mechanism (36-38)

In vivo murine
Suppressed via adrenergic and sympathetic neurotransmitters, glucocorticoid, dopaminergic, and peptide Y signalling

In vitro murine
Reduced NFAT binding to DNA and decreased IL2 production (55)

In vivo murine
Reduced cellularity and induced Fas (56) Mononuclear cells, including PBMC

In vitro human
Induced apoptosis via Fas (56)

In vitro human
Induced apoptosis via NO (58, 59)

In vitro human/ murine
Th2 switch with increased IL4 and IL5 and decreased IL2 and IFN g

In vitro & in vivo
Reduced phagocytosis due to reduced SO anion directly via MOR (62-66)

In vitro & in vivo
Inhibited phagocytosis by MOR and D2OP in a dose-dependent manner by inhibiting actin polymerisation via the inhibition of Rac1-GTPase and p38 MAPK (51)

Ex vivo murine
Reduced respiratory burst activity (morphine-stimulated NO release mediated by an M3OR subtype expressed on the surface of monocytes, in contrast to fentanyl) (67, 68)

In vivo murine
Reduced sticking and rolling along the blood vessels (70)

T lymphocytes A In vitro murine
Reduced response to ConA via a centrally acting mechanism (71, 72)

In vivo murine
Recused calcium reflux in CD4+ via a glucocorticoid-mediated mechanism (73)

In vitro murine
Reduced proliferation stimulated by IL4 and anti-IgM via a centrally acting mechanism (74-76)

In vivo murine
Inhibition of calcium mobilisation is an early event in opiate-induced immune suppression. (

Ex vivo human
Reduced SO production (77) of morphine exposure extend to inhibiting chemotaxis and multiple cellular functions in macrophages, including respiratory burst activity, phagocytosis, and colony formation (Table 2).Suppression of the critical immune mediators such as tumour necrosis factor (TNF a), interferon (IFN g), and nitrous oxide (NO) produced by macrophages was also reported in multiple studies (Table 3).Figure 1 illustrates the immune pathways affected during the co-stimulation of macrophages by mycobacterial antigens and morphine.Suppression of the neutral killer (NK) cell by morphine is modulated via direct and centralised mechanisms.Lymphocytes are inhibited by multiple means, including reduced cytotoxicity and altered CD4/CD8 cell ratios.

The role of TLR during the interaction with mycobacterial antigens and morphine
TLRs are found in various immune cells and play a critical role in recognising molecular patterns of pathogens to trigger the immune system.Multiple TLRs, including TLR2, TLR4, TLR8, and TLR9, interact with various mycobacterial antigens.Morphine interacts with TLR2, TLR4, and TLR9, and costimulation by morphine and mycobacterial antigens may lead to complex immune effects downstream (49,(101)(102)(103)(104)(105).Multiple mycobacterial antigens interact with immune cells via TLR-dependent and independent mechanisms.Figure 1 illustrates the co-binding of mycobacterial antigen and opioids with the TLR4 and its adaptor proteins.Rapidly growing, non-pathogenic mycobacteria containing AraLAM in their cell walls activate CD14 cells expressing TLR2 and macrophages.In contrast, slowgrowing pathogenic MTB containing ManLAM has shown a relative inability to activate macrophages independent manner, potentially contributing to their virulence (Figure 2).However, other soluble and cell wall-associated mycobacterial antigens distinct from LAM can mediate immune cell activation via TLR.For example, a soluble heat-stable and protease-resistant factor mediates TLR2-dependent activation of immune cells, whereas a heat-sensitive cell-associated mycobacterial factor mediates TLR4dependent activation of them.(106-112) Interestingly, induction of adaptive T cell response in TB does not require TLR2/4/9.In TLR2/ 4/9-deficient mice, mycobacterial replication is controlled by TLRindependent mechanisms to induce an adaptive T-cell response (113).
Morphine interacts with TLR2, TLR4, and CD14 cells, causing inhibitory effects.(Figures 2, 3) These effects of morphine are exerted on different immune cell types interrupting their functions, which are vital for the immune defence against TB.Further, endomorphin-1, the endogenous form of opioids, has been shown to down-regulate TLR expression as a part of the negative feedback control.Consequently, external opioids, when strongly influencing the same pathway, may contribute to impaired and delayed antigen processing.(114) Consequently, the cells' capacity to interact with mycobacterial antigens and trigger a protective immune response is ultimately reduced.

Cross-talk between receptors
Figure 3 summarises the cross-talk between OR, TLR, and nucleotide-binding and oligomerisation domain-like receptors (NLR) 2 via multiple cytosolic second messengers during the interaction with morphine and mycobacterial antigens.In the central nervous system, cross-taking between OR and TLR share common cytosolic molecules such as MAPK, b-arrestin-2/TRAF6 complex, and the DNA-binding protein HMGB1.(115) NLR2 on the immune cells interacts directly with mycobacterial antigens and cross-talks with TLR 2/9 to modulate the immune response.(61) Cross-talk between MOR and TLR in cancer models has shown decreased NK cytotoxicity, decreased leucocyte migration, suppression of mast cell recruitment, and the induction of M2 cell polarisation, which may contribute to the immune impairment in TB (116).Further exploration of the role of these compounds within the immune system in modulating cellular function is required.

Variations in the antigenic stimulation
Co-stimulation of TLRs with antigenic material of a pathogen, together with morphine, has shown entirely different effects than the binding of either alone.Extensive in vitro and in vivo studies have consistently shown that morphine binding to TLR4 triggers a proinflammatory cytokine response downstream.In contrast, its binding to OR (opioid receptors) elicits an anti-inflammatory response.(104,117) We hypothesise that opioid compounds interact with TLR as other natural compounds, modulating the host immune response, and it needs direct testing of this in preclinical models in the presence of tuberculosis antigens.(Figure 3).
Morphine causes an antiinflammatory response in dendritic cells (DC) cells via TLR2 and NLR2 when co-stimulated with S. pneumoniae, in contrast to the proinflammatory response induced by S. pneumoniae alone (Figure 3).( 49) Morphine has also been shown to inhibit the TLR9 pathway when co-binding with HIV, promoting its replication in macrophages.(105) Similar variations of the immune effects have been observed with morphine and mycobacterial antigens in preclinical studies.Plasmacytoid DC expresses TLR9 in both humans and mice.(118-120) M. tuberculosis and morphine cotreatment have significantly upregulated TLR9 expression in mice.Its role is more proinflammatory, enhancing the levels of critical cytokines including TNF a, (interleukin) IL1b, and IL6, which contrasts with the antiinflammatory response exerted by TLR2/4 when costimulated by the same.(20,121) This contracting proinflammatory action of TLR9 compared to other TLRs indicates the downstream receptor action heterogeneity, possibly explained by the unique binding of TB antigens with TLR9.

Immunomodulatory effects of morphine in the immunopathogenesis of tuberculosis
In the immunopathogenesis of tuberculosis, morphine exerts immunomodulatory effects, as depicted in Figure 4.The acquisition of TB bacilli occurs through the inhalation of respiratory droplets containing the organism.Morphine's influence leads to the suppression of NK cells and DC, as highlighted in Table 1.Consequently, this suppression can impair the initial defence against TB bacilli, including nonspecific killing and antigen presentation by these cells, ultimately increasing the host's susceptibility to TB infection.

Effects on the TB granuloma
A TB granuloma is a unique pathological entity comprising activated macrophages, monocytes, DC, neutrophils, and T lymphocytes (Figure 4).An established granuloma contains infected activated macrophages and epithelioid cells forming a central necrotic core and activated macrophages and layers of CD4+ and CD8+ T cells defining a dense cellular wall encircling the necrotic core.(122, 123) It is a dynamic structure that controls two processes: the induction of apoptosis of infected macrophages and the recruitment of uninfected macrophages by creating a chemotactic gradient (123).
Granuloma formation is triggered by the mycobacterial virulence factor ESX-1 (124) It triggers matrix metallopeptidase (MMP)9 secretion by the epithelial cell matrix surrounding a granuloma (Figure 4).( 125) It potentially induces the chemotaxis of macrophages via (chemokine ligand (CCL)7, a substrate for MMP9 produced by macrophages.(126)(127)(128)(129) Both wellcoordinated processes of new macrophage recruitment and infected macrophage apoptosis are essential to maintain the immune integrity of TB granulomas.Morphine decreases the levels of MMP9 and increases the tissue inhibitor of MMP1, dysregulating this process (Figure 4) (108).

Macrophages
Macrophages, crucial in granuloma formation, face inhibition through various mechanisms induced by morphine (Figure 2).When infected with M. tuberculosis, macrophages experience heightened caspase-8-dependent apoptosis due to TLR2 signalling.However, mycobacteria take advantage of this situation during the initial stages of infection, as they depend on macrophages to penetrate deeper tissues and subsequently undergo apoptosis to expand within the granuloma.(123, 130) Morphine causes enhancement of TLR9-induced apoptosis of macrophages by stimulating TLR9 signalling, and multiple chemical mediators also potentiate apoptosis (Figures 1, 2).(20,59) The impact of the induction of apoptosis by morphine depends on the exact stage of the infection.
MOR located on macrophages inhibits chemotaxis, which supports the notion of an antiinflammatory role of MOR.(131) Morphine's effects on phagocytosis were variable, with inhibition observed through a naloxone-reversible mechanism.Mycobacteria employ the cell wall-associated lipid Phthiocerol dimycocerosate (PDIM) to conceal underlying pathogen-associated molecular patterns (PAMPs), effectively evading the recruitment of microbicidal macrophages via TLR-dependent pathways.Additionally, a structurally related molecule called Surface-associated Phenolic glycolipid induces the expression of CCL2, leading to the recruitment and infection of CCR2-expressing macrophages.Morphine suppresses CCL2, counteracting this pathway and suppresses multiple chemokines important in chemotaxis (Table 3).

Lymphocytes
Studies have shown that the reduction in total T cell counts and altered CD4/CD8 cell ratios are caused by morphine.Suppression of IL2 levels by morphine leads to a drop in T cell count (Table 2).CTLA-4 and PD-1 are two members of the CD28 family of receptors involved in T-cell inhibition by morphine.(132)(133)(134) In murine studies, MOR agonists have been shown to upregulate the expression of MOR, DOR, CD28, CTLA-4, and PD-1, which suppresses T-cell response.However, chronic opioid use has led to increased expression of CTLA-4, with unchanged PD-1 expression favouring an anti-inflammatory response among humans.(135) Morphine further triggers the Th2 switch, which may impair the cytotoxic potential of T cells against TB bacilli (Figure 4).Further studies are required to explore variable immune effects caused by different opioids on T cells.

Neutrophils
Neutrophils are abundant in both early granulomas and late cavitary granulomas.(32, 136) They exhibit reduced NADPH oxidase-dependent mycobacterial killing when they ingest mycobacteria.However, their role in mediating the clearance of infected, dying macrophages appears to be host-protective.This mechanism lowers the mycobacterial load and reduces intercellular spread into uninfected macrophages.(123) The inhibition of neutrophils and IL8 secretion by morphine may lead to reduced neutrophil-mediated killing, increasing the risk of TB bacilli dissemination (Table 3).

Interferons
IL17 recruits Th1 cells that secrete antigen-specific IFN g, inhibiting MTB growth.Th1-mediated IFN g is the critical chemical controller in granuloma formation.M. tuberculosis induces an IFN g response through TLR9's action.(21) Once stimulated by TB antigens via TLRs, macrophages and DCs secrete cytokines, including IL-12 and IL23, to induce IFN-g production by T and NK cells.IFN-g increases phagocytosis, phagolysosomal fusion, oxidative burst, and other nonoxidative mechanisms.(137) For an effective T helper 1 (Th1, IFN-g producer cells) differentiation, costimulation (e.g., CD40L-CD40 and CD28-CD80/CD86 interactions) and NF-kB dependent signalling are essential.(138) IFN g deficiency leads to a failure in granuloma formation, with subsequent infiltration of neutrophils leading to cellular necrosis.(122, 123) Bloom et al. have shown that macrophage-induced NO is the primary bactericidal mechanism of macrophages.It is established that IFN g is an inducer of macrophage inducible NO synthase that leads to the production of NO (Figure 2).(139) IL1b is another mediator induced by mycobacterial antigens, which upregulate iNOS and subsequent NO production.NO-mediated killing by macrophages is the primary mechanism for controlling mycobacterial replication.A hypothesis can be proposed that the inhibitory effects of morphine on IFN g, NO, and IL1b may lead to a dysregulation of this process, ultimately exerting negative impacts on granuloma formation (Figure 4; Table 2).Therefore, the suppression of INF-g by morphine induces multiple significant negative implications on the immune defence against TB.
The role of IFN a/b on TB immunity is highly variable in contrast to the protective role of IFN-g.Type I IFNs (IFN a/b) are potent inhibitors of IL-12 production by macrophages, which induces IFN-g.( 140) Conversely, they induce IFN-g production by T and NK cells in an IL-12-independent way.( 141  Multiple studies have reported that the induction of Type I IFNs precedes the onset of clinical tuberculosis.(145,146) MTB inhibits the production of IFN a/b in response to TLR9 signalling.Morphine further suppresses this, producing complex effects requiring further characterisation in controlled studies (Table 2; Figure 3).

Tumour necrosis factors
M. tuberculosis-induced TNF a production appears to be controlled via TLR2.(122) Both TNF a deficiency and excess can lead to granuloma necrosis.(147,148) The TNF a signalling deficiency in mice produced disorganised tuberculous granulomas.(149, 150) Deficient TNF a signalling increases intra-macrophage mycobacterial load and accelerates the formation of disorganised granulomas, ultimately leading to granuloma necrosis (130, 151).Morphine has been shown to suppress TNFa levels in many studies, which may enhance the progression locally (Tables 2, 3).It may further affect disorganised secondary granuloma formation in distal organs, leading to disseminated disease.

Interleukins and chemokines
Morphine upregulates CCR expression while downregulating CCL levels, causing a net deficiency of CCL.(Figure 2) (21) IL6 stimulates macrophage and cytotoxic T-cell differentiation.At the same time, IL10 inhibits proinflammatory cytokines, blocks the generation of ROI and NOI, blocks antigen processing and presentation in different APCs, and diminishes T-cell responses.IL12 is a crucial cytokine in developing and maintaining type 1 cellular response in MTB infection.IL12 binds to its receptor IL12R-b2 and activates the JAK-STAT pathway, inducing IFN g to differentiate CD4+ T cells into Th1 effectors.Preclinical evidence has shown that IL-12 p40−/− deficient mice could not control bacterial growth, which appeared to be linked to the absence of both innate and acquired sources of IFN-g.( 152) This shows the central role played by IL12 in the defence against TB infection.IL23 induces IL17 production by memory T cells, creating an inflammatory response by Th17 cells, and it generates protective cellular responses.Morphine blocks the synthesis of all these vital mediators and damages the chemical coordination in the immune defence against TB.Moreover, IL12 induces inflammation by suppressing TGF ß and stimulating NK cells, contributing to increased CCL2 and CCL3 levels.(153) However, this effect may be counteracted by overall anti-inflammatory actions caused by morphine and the virulence mechanisms of mycobacterial antigens.(Table 3, Figure 4).

Conclusion
Chronic morphine administration causes suppression of multiple protective immune pathways vital in the defence against MTB.Multiple cellular receptors in immune cells, including OR, TLR2, and NLR 2, play critical roles in immunosuppression via complex intracellular cross-talk.Various cell types and their mediators involved in granuloma formation are inhibited by morphine via multiple mechanisms.This leads to a state of immunodeficiency that likely contributes to the reactivation, progression, and dissemination of MTB.Further studies are required to characterise potential therapeutic immunomodulatory targets in chronic opioid users at risk of infection with/reactivation of MTB.

Limitations
In vitro and in vivo preclinical studies assessing the immunomodulatory properties of opioids have been mostly limited to the testing of morphine.Considering the wide structural diversity and functional variation of opioids, direct testing of other categories of opioids and their antagonists is needed to delineate further the postulated mechanisms of the immunopathogenesis of TB potentiated by chronic opioid use.The duration and dosage of morphine use in patients may vary widely, with the added effect of the landscape of genetic heterogeneity across cultures, the impact of this review may be biased towards findings reported from the majority of studies originating from the Western, further studies are desperately needed from third-world nations investigating this phenomenon associating opioid use and the predisposition to TB. Chronic opioid use is associated with confounding factors, socioeconomic deprivation, malnutrition and infections associated with IVDU which may contribute to immune suppression.

FIGURE 1
FIGURE 1 Illustration of the co-binding of M. tuberculosis (MTB) antigens and opioids with TLR4.Illustration of the co-binding of MTB antigens and opioids with TLR4.Co-stimulation of the TLR4 leads to MyD88dependent and independent activation, leading to NFkB-dependent and independent mechanisms affecting chemical mediators downstream.[TLR, toll-like receptor; NF-kB, Nuclear factor-kB; TIRAP, Toll-interleukin-1 Receptor (TIR) domain-containing adaptor protein; TRIF, TIR-domain-containing adaptor-inducing interferonb; TRAM, TRIF-related adaptor molecule].

FIGURE 2
FIGURE 2 Illustration of the interactions by tuberculous antigens and morphine with the cell surface and intracellular receptors of a macrophage.The interaction between tuberculous antigens and morphine within macrophages.Morphine (M) and Mycobacterial antigens engage with TLR 2/4/9, while Mycobacterial antigens interact with NLR, MR, and Dectin-1.M enhances Mycobacterial antigen virulence by inhibiting TLR 2/4 and OR.Morphine influences various macrophage functions: (1) Desensitizing multiple CCRs via MOR and DOR.(2) Inhibiting FcR-mediated apoptosis via MOR, DOR, and KOR.(3) Enhancing TGF-induced apoptosis.(4) Impairing NO and SO synthesis, respiratory burst activity, and bacilli killing.NLR2 augments TLR 2/4 actions through cross-talk.(6) M inhibits NFkB-mediated cytokine and chemokine synthesis via OR-TLR cross-talk.TLR9 elicits a proinflammatory response with Mycobacterial antigens and M, while TLR2/4 induces an anti-inflammatory response.(7) MR, present in AM, regulates protective macrophage responses.MTB or ManLAM upregulates PPARg via MR, increasing IL8, COX2, and PGE2.ManLAM generates an antiinflammatory response, inhibiting proinflammatory TNF and IL12 while inducing immunosuppressive IL10 and TGF b.ManLAM signalling via TLR2 and TLR4 triggers chemokine secretion in monocytes, with M exerting inhibitory effects via TLR 2/4 (10).Dectin-1, in combination with TLR2, induces TNF production in macrophages, particularly in attenuated MTB strains (11).LM blocks TLR2-induced TNF biosynthesis, permitting MTB to evade the host immune response.Antigen-specific variations are noted, with M-induced immune suppression amplifying antigen virulence mechanisms.(A more detailed version of this figure legend is provided in the Online Data Supplement).
) IFNa/b is shown to reduce monocyte viability.compromises their bacteriostatic activity and antigen presentation ability.(142) Type I IFNs have been used as an adjunctive therapeutic agent for PTB patients harbouring multi-drug resistant MTB strains.(143,144)

TABLE 1
Summary of the epidemiological data assessing tuberculosis among drug users.

TABLE 2
Effects of morphine on the immune cells.

TABLE 3
Morphine effects on the chemical mediators.